This is sure to be the most often repeated brutal detail from Jon Krakauer’s new book: that a Mormon fundamentalist named Dan Lafferty spoke briefly to his 15-month-old niece on July 24, 1984, just before he killed her with a 10-inch boning knife. Lafferty explains to the author from his permanent home in a Utah state prison, ‘‘I told her: ‘I’m not sure what this is all about, but apparently it’s God’s will that you leave this world. Perhaps we can talk about it later.’’’

‘‘Under the Banner of Heaven’’ wants to talk about it now. It wants to link the double murder of Erica and her mother, Brenda, committed by two of her brothers-in-law, to a larger and no less bloody tableau of Mormon extremism throughout American history. In collecting evidence, Krakauer ventures out to a lunatic fringe of polygamous self-appointed prophets, where the Mormons and the Martians are almost interchangeable. He is even able to connect them to the fanatics found on Mount Everest in his enormously successful ‘‘Into Thin Air,’’ a bravura display of his nonfiction storytelling skills. While Krakauer is clearly interested in obsessive, risk-taking mavericks (and described them so well in the act of mountain-climbing), this book does not evolve naturally from that one.

Long underwear is a common factor, even if it is worn less understandably by devout polygamists in the desert than by freezing mountaineers. But ‘‘Under the Banner of Heaven’’ understands this as freakishness rather than fervor. Echoing Mark Twain’s opinion that ‘‘The Book of Mormon’’ is ‘‘chloroform in print,’’ this book provides more voyeuristic astonishment than curiosity or understanding.

For readers who know nothing of even the mainstream Mormon past, Krakauer presents details that sound stranger than fiction: how the angel Moroni told Joseph Smith Jr. of 1,400-year-old solid gold plates bearing scripture, buried in upstate New York; how the Mormons fled from a place called Nauvoo (in Illinois); how a ‘‘peep stone’’ can provide magical visions; how even the deceased can be inducted into the fold.

This history is complex enough for the book to warrant many long footnotes, enough to indicate organizational difficulties. It becomes even more tangled in investigating fundamentalist sects and splinter groups devoted to plural marriage as a holy principle and to the abundance of dangerous self-proclaimed prophets like Dan and Ron Lafferty. Ron, who once tried to kill Dan because God told him to, also believes that God has said to him: ‘‘And surely I will fulfill all my promises unto my servant Ron.’’

Shifting bumpily between chapters devoted to historical events and present-day loose cannons, Krakauer has also shoehorned in a chapter about the abduction of Elizabeth Smart at age 14 and a covetous polygamist, Brian David Mitchell, the man charged in the case. This section is more obligatory than revealing, but it underscores a pattern of sexual abuse and incest that runs through this material. Among many examples is that of Kenyon Blackmore. One of his daughters says she was raped by him and says he means to ‘‘marry’’ her sisters when each one turns 12.

Not surprisingly, Krakauer has ruffled feathers in the Church of Jesus Christ of Latter-day Saints, which disavows such behavior. He has also prompted what is sure to be renewed in terest in Mormon life. Sally Denton’s ‘‘American Massacre’’ overlaps with ‘‘Under the Banner of Heaven’’ in investigating the 1857 Mountain Meadows Massacre, which implicated Mormons in the killing of pioneers from Arkansas.

Dorothy Allred Solomon, who comes from a prominent line of polygamists, puts a more human face on this subject by describing her own experience as the 28th of her father’s 48 children. She was raised by seven mothers and remembers a time ‘‘when Daddy went away to college,’’ actually when he served five years in prison for illegal cohabitation.

If Solomon’s story sometimes sounds like the stuff of daytime television talk shows, it is. She regrets having been bumped from a date on ‘‘Donahue’’ by Winnie Mandela and has appeared with Sally Jessy Raphael. But it is a remarkable tale. She grew up with the knowledge that the mere fact of her birth could mean more prison time for her father, Rulon Allred (a physician who is also mentioned by Krakauer). She hid with her family to avoid government raids and tells how one faction wound up in a remote spot, living on nothing but carrots.

And of course she witnessed the everyday reality of a polygamous family. ‘‘You promised us you would marry only virgins,’’ she says the mothers insisted, when Allred wondered whether it was his spiritual duty to take more wives. (He did. He wound up with 16.) Allred’s assassination in 1977 by a rival fundamentalist group fits into the bloody tradition that all these books describe.

Solomon is best when not being lyrical. (‘‘The truth is cold, a peach frozen in January.’’)

For the most part Solomon is outspoken and frank, free of the dissembling to avoid prosecution that she calls ‘‘practicing Mormon logic.’’ To Krakauer, this is ‘‘lying for the Lord.’’